Basic: refactor memory layout.

Basic: refactor memory layout.

Use a one dimensional array for the Z% value array. This enables
lists, vectors, environments and metadata pointers to all save off
1 word (2 bytes) of space.

Split the memory init and functions into mem.in.bas.

In addition, change type 14 to be metdata rather than any type 16-31.

This change saves about 560 bytes (no second array dimension
subscripts) and reduces Z% value usage by 10%-15%.

Bump the number of Z% words by 200 (to 8591). This enables
self-hosting up to step7 (without step8-stepA functions in core.mal).